Problem with mails

I have configured linux sendmail so that it can send mails from the server to the exchange accounts. For this I have edited sendmail.cf file and added the smtp server address in the like :DSxyz.com

After that I restarted the sendmail server.After this changes i was able to send mails from the server however when I try to reply to these mails they are not getting delivered. Could you please give me any idea what can be the problem.